Resilient conductive coated foam member and electromagnetic shield employing same



May 27, 1969 D. ZULAUF 3,446,906

RESILIENT CONDUCTIVE COATED FOAM MEMBER AND ELECTROMAGNETIC SHIELDEMPLOYING SAME Filed May 17, 1967 FIG. I FIG. 2

DIETER ZULAUF INVENTOR BY BUCKHO/PN, BLORE, KLAROU/ST 8 SPAR/(MNATTORNEYS United States Patent US. Cl. 17435 8 Claims ABSTRACT OF THEDISCLOSURE The subject matter of the present invention relates generallyto a porous, resilient, electrically conductive member formed of a bodyof open-celled plastic foam having a coating of electrically conductivematerial provided throughout such foam body on the surface of the foamcells. This resilient, conductive foam member is employed as anelectromagnetic shield to prevent electrical signal radiation frompassing between two metal plates between which such conductive member iscompressed. A number of different methods are employed to form thepresent resilient, conductive member including vacuurn vapor deposition,spraying, dipping or electroplating. A polyurethane plastic foam havinga plurality of interconnected open cells is employed to provide a foambody which is suificiently porous to enable fluid to pass through suchbody so that all of the surfaces of the cells throughout the body arecoated with nickel or other metal.

Background 0 the invention The resilient electrically conductive coatedfoam member of the present invention is especially useful as anelectromagnetic shield to prevent electrical signal radiation frompassing therethrough. The electromagnetic shield may be provided by agasket of such conductive coated foam compressed between two metalmembers. In addition, the resilient conductive foam member can bemounted over an opening in the housing of an electronic instrument, suchas a cathode ray oscilloscope, through which air is circulated by a fanmounted within the housing for cooling purposes, in order to employ suchfoam member as a dust filter and as an electromagnetic shield to preventexternal radio frequency interference from entering the housing throughsuch opening. The resilient conductive foam member can be used merely asan electrical connector between two metal plates between which it iscompressed. It can also be employed in an electrostatic precipitator asan air filter to which an electrical potential is applied to remove dustand other charged particles from the air. The conductive foamelectromagnetic shield can also be employed in many other differentplaces on an oscilloscope such as around the knob openings on the frontpanel through which the knob shafts extend, and over the fan opening atthe rear of the instrument.

Previously, plastic foam material has been employed in electromagneticshields with a conducting layer or plate provided only on the outside ofsuch shield, as shown in US. Patent 2,870,439 of H. E. Stinehel-fer andUS. Patent 3,147,336 of H. G. Mathews. These prior art shields have thedisadvantage that they are not resilient and are not sufficientlyconductive throughout to enable them to be employed as gaskets or othermembers which are compressed between two metal plates and still provideproper shielding. These disadvantages are overcome by the resilient,conductive coated foam member of the present invention which issufliciently porous to enable fluid to be transmitted therethrough sothat the conductive coating is provided on the surface of all of thefoam cells throughout the member.

Summary of the invention It is therefore one object of the presentinvention to provide a resilient, electrically conductive coated plasticfoam member which is suificiently porous to enable the passage of fluidtherethrough.

Another object of the invention is to provide an electromagnetic shieldof electrically conductive coated plastic foam material which is porous,resilient and conductive throughout which is capable of high attenuationof electrical signal radiation.

A further object is to provide an improved gasket of resilientelectrically conductive plastic foam.

An additional object of the present invention is to provide a porous,electrically conductive member which includes a body of plastic foamhaving .a plurality of interconnected open cells which provide asufliciently porous structure to enable fluid to pass through such body,and a coating of conductive material on the surface of the cells of thefoam throughout the body to provide an inexpensive, porous, lightweightmember of high conductivity.

Brief description of drawings Additional objects and advantages of thepresent invention will be apparent from the following detaileddescription of the preferred embodiments thereof, and from the attacheddrawings, in which:

FIG. 1 is an elevation view of an electromagnetic shield gasket made ofa porous, resilient, conductive coated plastic foam material inaccordance with the present invention;

FIG. 2 is a vertical section view taken along the line 2-2 of FIG. 1;

FIG. 3 is an enlarged view of a portion of the conductive coated plasticfoam member of the present invention;

FIG. 4 is an enlarged section view taken along the line 44 of FIG. 3 ofone element of the conductive coated plastic foam;

FIG. 5 is a section view showing the use of the gasket of FIG. 1 as ashield surrounding an opening in a metal housing and compressed betweena closure plate and such housing to prevent electrical signal radiationfrom entering the housing through such opening; and

FIG. 6 is a section view showing the present conductive coated foammember employed as an electromagnetic shield and an air filter over theopening in a metal housing through which air is circulated by a fanwithin such housing.

Detailed description of preferred embodiments As shown in FIGS. 1 and 2the resilient, conductive coated plastic foam member of the presentinvention can be made in the form of a gasket 10* having a plurality ofbolt openings 12 provided through such gasket at positions placed aroundits periphery and having a large central opening 14. As shown in FIG. 2the gasket 10 is a thin, flexible sheet of plastic foam material. Theplastic foam is an open-celled, resilient polyurethane, which is coatedthroughout with a conducting material, such as gold, silver, aluminum ornickel in any suitable manner, such as by vacuum vapor deposition,liquid spraying or dipping, or electroplating.

As shown in FIGS. 3 and 4, the plastic foam has a plurality ofinterconnected open cells 16 in order to provide a body which issuificiently porous to enable the transmission of fluid, includingliquid or gas, therethrough. This is necessary for coating all of thesurfaces of the foam cells through the body with conductive material. Asshown in FIG. 4, each of the plastic elements 18 of the foam cells isprovided with a coating 20 of conducting material on the surfacethereof. It should be noted that while a single-layer conductive coating20 is shown, multilayer coatings can also be employed as is necessarywith electroplating since the first layer must be provided by dipping inorder to provide a conductive surface on the insulating foam which formsthe cathode on which the electroplating takes place. For example, thecoating 20 may be formed by an inner layer of copper provided by dippingand an outer layer of nickel electroplated onto such copper layer.

While the size of the open cells or pores in the plastic foam may varybetween approximately 30 to 90 pores per inch, it has been found that apolyurethane foam having 45 pores per inch enables sufficient drainageto remove excess copper dip solution while retaining enough conductivematerial to provide a uniform conductivity throughout the foam body. Itshould be noted that one of the problems with spraying a metal coatingis that the metal sometimes oxidizes, providing an insulating ratherthan a conductive coating. Therefore, the spraying may have to becarried out in an inert atmosphere. Also some metal coatings may be toobrittle to provide the necessary resiliency or too fragile so they arereduced to powder by compression of the plastic foam during use.

Vacuum vapor deposition can be employed to provide a conducting coatingof gold, silver, copper or aluminum by providing the metal in a layer ona tungsten heater filament next to which is placed the plastic foam in avacuum chamber which may be evacuated to approximately 2x10 torr. Thetungsten filament is then heated to the evaporation temperature of themetal to cause the metal vapor to coat the plastic foam. With thickpieces of foam it may be necessary to vacuum evaporate from both sidesof the sheet from several different places in order to cause the metalvapor to penetrate the foam sufficiently to provide a substantiallyuniform conductive characteristic throughout the foam. Therefore, forsheets of large area or thickness, it is preferable to use anelectroplating technique mentioned above.

A sulfamate nickel electroplating technique which has been employed tocoat a sheet of polyurethane foam approximately 1.25 square feet in areais hereafter described. First, the polyurethane foam sheet is cleaned ina neutral detergent and rinsed with cold water. Then the foam sheet isdipped in a sulfuric acid solution and rinsed in cold water. After thisit is then dipped in a solution of hydrochloric acid. Next the foamsheet is dipped in a catalyst solution of stannous chloride,hydrochloric acid and palladium chloride. The stannous chloride reactswith the palladium chloride to form stannic chloride and palladium whichis deposited as a thin film of palladium on the surface of the cells inthe polyurethane foam. This palladium film is necessary to enable theintermediate layer of copper to adhere to the polyurethane in a mannerhereafter described. The polyurethane foam sheet is then rinsed in coldwater and dipped in a bath of conventional accelerator material whichincreases the speed with which the copper deposits out of electrolesscopper olution onto the surface of the plastic foam. After rinsing incold water, the plastic foam sheet is next dipped into the electrolesscopper solution which includes copper sulfate, and remains in suchsolution for approximately five minutes to provide a copper coating ofapproximately 90 microinches thick. During this time the palladiumreplaces the copper in the copper sulfate to form palladium sulfate anda layer of copper on the plastic foam because palladium is lower in theelectromotive series than copper.

After this copper coating, the coated foam sheet is rinsed in cold waterand dried by blasts of air emitted from an air gun at approximately 100lbs. per square inch pressure which removes any excess copper solutionre maining in the pores of the foam sheet after drainage. The coppercoated foam sheet is then mounted in a frame 4 or rack and dipped into asolution of sulfuric acid and rinsed with cold water. Next the coppercoated foam sheet is placed in a bath of sulfamate nickel platingsolution and electroplated with nickel over the copper layer whichserves as the cathode. The plating operation takes place forapproximately one minute at a current density of 40 amperes per squarefoot or a total amperage of 50' amps in order to produce a layer ofnickel approximately 30 microinches thick on the surface of the copperlayer. After plating, the nickel coated foam member is then rinsed incold: water and air-dried in an oven at Fahrenheit. Finally, the nickelcoated foam member is removed from the support frame or rack and cut tothe desired configuration such as the gasket of FIG. 1.

As shown in FIG. 5, the porous, resilient, conductive coated foam gasket10 of FIG. 1 can be employed as an electromagnetic shield around anopening 22 in an aluminum housing 24 of a cathode ray oscilloscope orother instrument. A plurality of threaded studs or bolts 26 are providedon the housing 24 to enable a cover plate 28 of aluminum to be fastenedto the housing by nuts 30 threaded onto studs 26 extending throughapertures 12 in the gasket 10 and similar apertures in the cover plate.As a result of the tightening of the nuts 30, the gasket 10 of thepresent invention is compressed between the cover plate 28 and thehousing 24 to effectively fill the space between such cover plate andhousing which is grounded. As a result the gasket 10 provides anelectromagnetic shield which prevents electrical signal radiation fromentering the housing through opening 22 by way of the space previouslyexisting between the cover plate and housing.

While the gasket embodiment of FIG. 5 is suitable when the opening 22 ismerely used as an access opening for repair of the instrument, it mayalso be necessary to provide the conductive foam member of the presentinvention as an integral sheet 32 extending across the opening 22 whenit is desirable to have air circulate through such opening. Thus a fan34 may be provided within the instrument for cooling purposes, as shownin FIG. 6. In this case, the cover plate 28 is replaced by a metal framemember 36 surrounding only the periphery of the opening 22. The frame 36is fastened to the housing 24 by nuts 30 threaded on to the bolts 26which extend through openings in the frame 36 and the conductive plasticfoam sheet 32 in order to compress such sheet only in the area aroundits outer edge. Since air must pass through the conductive foam sheet 32in the region over the opening 22, it acts as a filter to remove dustwhile at the same time functioning as an electromagnetic shield toprevent electrical signal radiation from entering the housing 24. Theshield 32 is grounded to the housing 24, which may be the housing of acathode ray oscilloscope containing electrical circuits which aresensitive to stray electrical fields.

An electromagnetic shield made in accordance with the above describednickel plating method is especially useful between two aluminum plates,because it does not chemically react with aluminum. One such shield wasoperated successfully as an electromagnetic shield against electricalsignal radiation having a frequency of 2000 megacycles per second whichit attenuated by 28.8 decibels. Thus, the electrical signal radiationtransmitted through the shield was 28.8 decibels less than the radiationtransmitted between the test panels when no shield was employed.

It will be obvious to those having ordinary skill in the art that manychanges may be made in the details of the above described preferredembodiment of the present invention without departing from the spirit ofthe invention. For example, other plastic foam materials can be employedthan polyurethane and other conductive coating materials can be usedthan nickel.

I claim:

1. A porous, resilient, electrically conductive member comprising:

a body of resilient plastic foam material having a plurality ofinterconnected open cells and being sulficiently porous to allow fluidto pass through said body; and

a coating of electrical conductive material provided throughout saidbody on the surfaces of the plastic elements of said body which form thecells of the foam material while maintaining the cells open so that saidconductive material is positioned inside the body as Well as on itsouter surface.

2. A member in accordance with claim 1 in which the plastic foammaterial is polyurethane.

3. A member in accordance with claim 2 in which the conductive materialis a metal taken from the group consisting of gold, silver, copper,aluminum and nickel.

4. A member in accordance with claim 1 in which the body is a thin,flexible sheet of plastic foam having approximately 45 pores per linearinch.

5. An electromagnetic shield including a resilient conductive member inaccordance with claim 1 and means for grounding said member to attenuateelectrical signal radiation tending to be transmitted through saidmember.

6. A shield in accordance with claim 5 in which the resilient conductivemember is compressed between two metal plates to attenuate electricalsignal radiation tending to be transmitted through the space betweensaid plates.

7. A shield in accordance with claim 5 in which the resilient conductivemember is supported over a hole extending through a metal plate toprevent electrical signal radiation from passing through such hole.

8. An electromagnetic shield including the resilient conductive memberof claim 1 formed as as a gasket having centrally located openingtherethrough much larger than the pores of the plastic foam.

References Cited UNITED STATES PATENTS 2,674,644 4/ 1954 Goodloe.

